Transposable element
semiparasitic DNA sequence, a major fraction of eukaryotic genomes From Wikipedia, the free encyclopedia
semiparasitic DNA sequence, a major fraction of eukaryotic genomes From Wikipedia, the free encyclopedia
A transposable element is often called a transposon. It is a sequence of DNA that can move to new positions in the genome of a single cell. The press sometimes call them jumping genes, but it is not correct to call them 'genes'.
Transposons were first found by Barbara McClintock while working on maize in the 1930s to 1950s. She discovered transposition in maize, but it took years before her work was understood. She received a Nobel Prize for her work in 1983.
Transposition can create significant mutations and alter the cell's genome size.
Transposons are only one of several types of mobile genetic elements. Transposons themselves are of two types according to their mechanism, which can be either "copy and paste" (class I) or "cut and paste" (class II).[1]
Class I (Retrotransposons, aka retroposons): They copy themselves in two stages, first from DNA to RNA by transcription, then from RNA back to DNA by reverse transcription. The DNA copy is then inserted into the genome in a new position. Retrotransposons behave very similarly to retroviruses, such as HIV.
Class II (DNA transposons): By contrast, the cut-and-paste transposition mechanisms of class II transposons do not involve an RNA intermediate.[2]: 284
Transposons are mutagens. They can damage the genome of their host cell in different ways:
Transposons can carry accessory genes, such as antibiotic resistance genes. They can be used to put a gene into the DNA of an organism. This has been done with fruit flies (Drosophila melanogaster) by putting the transposon into the embryo.
Transposons are found in many forms of life. They may have arisen independently many times, or perhaps just once and then spread to other kingdoms by horizontal gene transfer.[11]
While some transposons may confer benefits on their hosts, most are regarded as selfish DNA parasites. In this way, they are similar to viruses. Various viruses and transposons also share features in their genome structures and biochemical abilities, leading to speculation that they share a common ancestor.
Excessive transposon activity can destroy a genome, which is lethal. Many organisms have developed mechanisms to inhibit them. Bacteria may delete transposons and viruses from their genomes; eukaryotic organisms use RNA interference (RNAi) to inhibit transposon activity.
In vertebrate animal cells nearly all the 100,000+ DNA transposons in a genome code for inactive polypeptides.[12] In humans, all of the Class I-like transposons are inactive. The first DNA transposon used as a tool for genetic purposes, the Sleeping Beauty transposon system, was a transposon which was resurrected from a long evolutionary sleep.[13][14]
Transposons may have been co-opted by the vertebrate immune system as a means of producing antibody diversity: The V(D)J recombination system operates by a mechanism similar to that of transposons. This is a system of three genes which get rearranged in the production of vertebrate lymphocytes. The system diversely encode proteins to match antigens from bacteria, viruses, parasites, dysfunctional cells such as tumor cells,[15] and pollens.
The final DNA sequence, and thus the sequence of the antibody, is highly variable, even when the same two V, D, or J segments are joined. This great diversity allows VDJ recombination to generate antibodies even to microbes that neither the organism nor its ancestors have ever previously encountered.
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